278 research outputs found
Anomalous Hall effect in granular ferromagnetic metals and effects of weak localization
We theoretically investigate the anomalous Hall effect in a system of
dense-packed ferromagnetic grains in the metallic regime. Using the formalism
recently developed for the conventional Hall effect in granular metals, we
calculate the residual anomalous Hall conductivity and
resistivity and weak localization corrections to them for both
skew-scattering and side-jump mechanisms. We find that, unlike for
homogeneously disordered metals, the scaling relation between and
the longitudinal resistivity does not hold. The weak localization
corrections, however, are found to be in agreement with those for homogeneous
metals. We discuss recent experimental data on the anomalous Hall effect in
polycrystalline iron films in view of the obtained results.Comment: published version, 10 pages, 6 figure
Enhancement of Superconductivity in Disordered Films by Parallel Magnetic Field
We show that the superconducting transition temperature T_c(H) of a very thin
highly disordered film with strong spin-orbital scattering can be increased by
parallel magnetic field H. This effect is due to polarization of magnetic
impurity spins which reduces the full exchange scattering rate of electrons;
the largest effect is predicted for spin-1/2 impurities. Moreover, for some
range of magnetic impurity concentrations the phenomenon of {\it
superconductivity induced by magnetic field} is predicted: superconducting
transition temperature T_c(H) is found to be nonzero in the range of magnetic
fields .Comment: 4 pages, 2 figure
ЦИКЛОКОНВЕРТОР С КОМБИНИРОВАННЫМ ЗАКОНОМ УПРАВЛЕНИЯ ДЛЯ СИСТЕМ ЭЛЕКТРОСНАБЖЕНИЯ АВТОНОМНЫХ ОБЪЕКТОВ
The present paper considers cycle-converter with complicated control law. The methodj complicated control law generating is presented. The scheme of power circuits is introduced. Volta and current curves for classical cycle-converter and cycle-converter with complicated control lawi discussed. Introducing of complicated control law improves output voltage quality, increa fundamental harmonic's amplitude and input power factor. The article is attractive in the field of i power electronics.В статье рассматривается циклоконвертер с комбинированным законом управления вентильными комплектами. Представлен способ формирования комбинированного сигнала управления. Приведена схема силовых цепей исследуемого преобразователя. Рассматриваются эпюры кривых выходного напряжения и тока для классической схемы циклоконвертера и для схемы преобразователя с комбинированным законом управления. Введение комбинированного сигнала управления повышает качество выходного напряжения, увеличивает амплитуду основной гармоники и входной коэффициент мощности. Статья представляет интерес для специалистов в области авиационной силовой электроники.В статье рассматривается циклоконвертер с комбинированным законом управления вентильными комплектами. Представлен способ формирования комбинированного сигнала управления. Приведена схема силовых цепей исследуемого преобразователя. Рассматриваются эпюры кривых выходного напряжения и тока для классической схемы циклоконвертера и для схемы преобразователя с комбинированным законом управления. Введение комбинированного сигнала управления повышает качество выходного напряжения, увеличивает амплитуду основной гармоники и входной коэффициент мощности. Статья представляет интерес для специалистов в области авиационной силовой электроники
Surface impedance of superconductors with magnetic impurities
Motivated by the problem of the residual surface resistance of the
superconducting radio-frequency (SRF) cavities, we develop a microscopic theory
of the surface impedance of s-wave superconductors with magnetic impurities. We
analytically calculate the current response function and surface impedance for
a sample with spatially uniform distribution of impurities, treating magnetic
impurities in the framework of the Shiba theory. The obtained general
expressions hold in a wide range of parameter values, such as temperature,
frequency, mean free path, and exchange coupling strength. This generality, on
the one hand, allows for direct numerical implementation of our results to
describe experimental systems (SRF cavities, superconducting qubits) under
various practically relevant conditions. On the other hand, explicit analytical
expressions can be obtained in a number of limiting cases, which makes possible
further theoretical investigation of certain regimes. As a feature of key
relevance to SRF cavities, we show that in the regime of "gapless
superconductivity" the surface resistance exhibits saturation at zero
temperature. Our theory thus explicitly demonstrates that magnetic impurities,
presumably contained in the oxide surface layer of the SRF cavities, provide a
microscopic mechanism for the residual resistance.Comment: 9 pages, 3 figs; v2: published versio
Intracoronary administration of metabolic agents in the treatment of acute coronary syndrome
The article presents the new possibilities of preventing the onset of irreversible myocardial damage during ischemia and effective approaches undertaken in the early reperfusion period (per - and postconditioning of the myocardium) through coronary the introduction of cardioprotective drugs. Practitioners need medicines, which have fewer complications and are universal this is by far the metabolic drugs. The author surveyed 300 patients (three groups) with acute coronary syndrome (ACS) hospitalized no later than 4 hours from the beginning of anginal attack. Patients in group I (n=109) in the prehospital phase was carried out by the system tromboliticescoy therapy (TLT) in combination with intravenous etilmetilgidroksipiridina succinate (200 mg), which the hospital performed mechanical recanalization and angioplasty of the infarct-related coronary artery (IRA) with intracoronary introduction of etilmetilgidroksipiridina succinate (200 mg). Patients in group II (n=101) etilmetilgidroksipiridina succinate (200 mg) primary intracoronary injected during the endovascular procedure. Patients III (control) group (n=90) was carried out only angioplasty of the IRA. Conclusions: the effect of restoration of coronary circulation set after conducted thrombolytic therapy with intracoronary administration of etilmetilgidroksipiridina succinate immediately after successful angioplasty of the IRA, which helped to restore adequate blood flow to prevent relapses and extension of the necrosis zone, transition to a higher angina functional classes
Hall Transport in Granular Metals and Effects of Coulomb Interactions
We present a theory of Hall effect in granular systems at large tunneling
conductance . Hall transport is essentially determined by the
intragrain electron dynamics, which, as we find using the Kubo formula and
diagrammatic technique, can be described by nonzero diffusion modes inside the
grains. We show that in the absence of Coulomb interaction the Hall resistivity
depends neither on the tunneling conductance nor on the intragrain
disorder and is given by the classical formula , where
differs from the carrier density inside the grains by a numerical
coefficient determined by the shape of the grains and type of granular lattice.
Further, we study the effects of Coulomb interactions by calculating
first-order in corrections and find that (i) in a wide range of
temperatures T \gtrsim \Ga exceeding the tunneling escape rate \Ga, the
Hall resistivity and conductivity \sig_{xy} acquire logarithmic
in corrections, which are of local origin and absent in homogeneously
disordered metals; (ii) large-scale ``Altshuler-Aronov'' correction to
\sig_{xy}, relevant at T\ll\Ga, vanishes in agreement with the theory of
homogeneously disordered metals.Comment: 29 pages, 16 figure
Excitonic condensation in a double-layer graphene system
The possibility of excitonic condensation in a recently proposed electrically
biased double-layer graphene system is studied theoretically. The main emphasis
is put on obtaining a reliable analytical estimate for the transition
temperature into the excitonic state. As in a double-layer graphene system the
total number of fermionic "flavors" is equal to N=8 due to two projections of
spin, two valleys, and two layers, the large- approximation appears to be
especially suitable for theoretical investigation of the system. On the other
hand, the large number of flavors makes screening of the bare Coulomb
interactions very efficient, which, together with the suppression of
backscattering in graphene, leads to an extremely low energy of the excitonic
condensation. It is shown that the effect of screening on the excitonic pairing
is just as strong in the excitonic state as it is in the normal state. As a
result, the value of the excitonic gap \De is found to be in full agreement
with the previously obtained estimate for the mean-field transition temperature
, the maximum possible value ( is the Fermi energy) of both being in
range for a perfectly clean system. This proves that the energy scale really sets the upper bound for the transition temperature
and invalidates the recently expressed conjecture about the high-temperature
first-order transition into the excitonic state. These findings suggest that,
unfortunately, the excitonic condensation in graphene double-layers can hardly
be realized experimentally.Comment: 21 pages, 5 figures, invited paper to Graphene special issue in
Semiconductor Science and Technolog
Excitonic condensation in a double-layer graphene system
The possibility of excitonic condensation in a recently proposed electrically
biased double-layer graphene system is studied theoretically. The main emphasis
is put on obtaining a reliable analytical estimate for the transition
temperature into the excitonic state. As in a double-layer graphene system the
total number of fermionic "flavors" is equal to N=8 due to two projections of
spin, two valleys, and two layers, the large- approximation appears to be
especially suitable for theoretical investigation of the system. On the other
hand, the large number of flavors makes screening of the bare Coulomb
interactions very efficient, which, together with the suppression of
backscattering in graphene, leads to an extremely low energy of the excitonic
condensation. It is shown that the effect of screening on the excitonic pairing
is just as strong in the excitonic state as it is in the normal state. As a
result, the value of the excitonic gap \De is found to be in full agreement
with the previously obtained estimate for the mean-field transition temperature
, the maximum possible value ( is the Fermi energy) of both being in
range for a perfectly clean system. This proves that the energy scale really sets the upper bound for the transition temperature
and invalidates the recently expressed conjecture about the high-temperature
first-order transition into the excitonic state. These findings suggest that,
unfortunately, the excitonic condensation in graphene double-layers can hardly
be realized experimentally.Comment: 21 pages, 5 figures, invited paper to Graphene special issue in
Semiconductor Science and Technolog
Excitonic condensation in a double-layer graphene system
The possibility of excitonic condensation in a recently proposed electrically
biased double-layer graphene system is studied theoretically. The main emphasis
is put on obtaining a reliable analytical estimate for the transition
temperature into the excitonic state. As in a double-layer graphene system the
total number of fermionic "flavors" is equal to N=8 due to two projections of
spin, two valleys, and two layers, the large- approximation appears to be
especially suitable for theoretical investigation of the system. On the other
hand, the large number of flavors makes screening of the bare Coulomb
interactions very efficient, which, together with the suppression of
backscattering in graphene, leads to an extremely low energy of the excitonic
condensation. It is shown that the effect of screening on the excitonic pairing
is just as strong in the excitonic state as it is in the normal state. As a
result, the value of the excitonic gap \De is found to be in full agreement
with the previously obtained estimate for the mean-field transition temperature
, the maximum possible value ( is the Fermi energy) of both being in
range for a perfectly clean system. This proves that the energy scale really sets the upper bound for the transition temperature
and invalidates the recently expressed conjecture about the high-temperature
first-order transition into the excitonic state. These findings suggest that,
unfortunately, the excitonic condensation in graphene double-layers can hardly
be realized experimentally.Comment: 21 pages, 5 figures, invited paper to Graphene special issue in
Semiconductor Science and Technolog
- …